1. Field of the Invention
The present invention relates to active matrix display devices comprising polycrystalline silicon thin film transistors and the manufacture thereof.
2. Background of the Invention
The high carrier mobility of polycrystalline silicon (poly-Si) relative to amorphous silicon (a-Si) makes it an attractive material for use in large area electronic devices such as active matrix liquid crystal displays (AMLCDs) and active matrix polymer LED displays (AMPLEDs). Conventionally, poly-Si films used for example in thin film transistors (TFTs) have been manufactured by solid phase crystallisation (SPC). This involves depositing an a-Si film on an insulating substrate and crystallising the a-Si film by exposing it to a high temperature for a prolonged period of time, that is typically a temperature in excess of 600° C. for up to 24 hours.
As an alternative, U.S. Pat. No. 5,147,826 discloses a lower temperature method of crystallising an a-Si film, comprising the steps of depositing a thin film of nickel for example on the a-Si film and annealing the film. The metal catalyses crystal growth at temperatures below 600° C. and also provides more rapid crystal growth than would otherwise occur. For example, a typical anneal using the method of U.S. Pat. No. 5,147,826 might be at around 550° C. for 10 hours. This represents an improvement over prior methods for at least two reasons: first, it enables low cost non-alkali glass substrates such as borosilicate to be used which would normally suffer glass compaction and warp at temperatures of 600° C. or more; and secondly, as the anneal duration is reduced, the manufacturing throughput rate is increased and therefore the associated manufacturing cost may be reduced. The contents of U.S. Pat. No. 5,147,826 are incorporated herein by reference.
Polycrystalline silicon material formed by a process in which a metal element is used to enhance the crystallisation process, as described in U.S. Pat. No. 5,147,826 for example, is referred to hereinafter as metal induced crystallisation poly-Si or MIC poly-Si.
In known active matrix display devices, the displayed image is created by an array of pixels, distributed in rows and columns over the “pixel area” of a substrate of the device. One or more TFTs are provided in each pixel to control the respective pixel. Integrated drive circuits comprising TFTs are provided in a “drive circuit area” on the same substrate, around the pixel area. Signals defining the images to be displayed are fed to the drive circuits which in turn send signals to the pixels to generate the images. As described in U.S. Pat. No. 5,756,364, it is desirable for the TFTs of the drive circuit area to have a high mobility, whilst in the pixel area, it is more important that the TFTs have a sufficiently small off-current and that the variation in off-current of the TFTs across the display is minimised. The contents of U.S. Pat. No. 5,756,364 are incorporated herein by reference.
U.S. Pat. No. 5,756,364 discloses that the crystallinity of MIC poly-Si may be improved by illuminating it with laser light. It states that TFTs formed in this way have a relatively high mobility of more than 100 cm2/Vs, making them suitable for the drive circuitry of an active matrix display. Such TFTs are said however to have large off-currents that greatly vary from one transistor to another, making them unsuitable for use in the pixel area of a display. It proposes the formation of MIC poly-Si only in the drive circuit area, followed by illumination of the drive circuit and pixel areas with different respective laser illumination energy densities, which forms poly-Si in the pixel area. The TFTs in the drive circuit area have the suitable properties noted above, whilst the TFTs created in the pixel area have a relatively small mobility of less than 20 cm2/Vs, but a small off-current variation. In order to restrict the formation of MIC poly-Si to drive circuit area, a silicon dioxide mask is formed over the pixel area prior to addition of a solution containing the metal element used to enhance the crystallisation process.